When electronic equipment is connected to long signal or power lines, antenna etc, it is exposed to transients generated by induction, caused by lightning or electromagnetic pulses (EMP). A surge arrester protects the equipment from damage by absorbing the energy in the transient or by connecting it to ground. Surge arresters are required to be self-recovering, able to handle repetitive transients and can be made fail-safe. An important property is the speed and selectivity of ignition, in other words, the surge arrester must function without delay and still not be so sensitive, that it is triggered by a normal communications signal. These properties should remain unchanged over time and irrespective of the ignition intervals. Further, a surge arrester should be suitable for mass production with high and uniform quality.
Gas-filled discharge tubes are used for protecting electronic equipment but are also frequently used as switching devices in power switching circuits, e.g. in automotive products such as gas-discharge headlights. Other application areas are tele- and data communications, audio/video equipment, power supplies, industrial, medical devices, security and military applications.
Early surge arresters comprised two solid graphite electrodes, separated by an air-gap or a layer of mica. These are, however, not comparable to the modern surge arresters with respect to size, reliability, performance and production technology.
A modern conventional surge arrester is the gas filled discharge tube, which may have one or several discharge paths or discharge gap and usually comprises two end electrodes plus optionally one additional electrode in the form of a center electrode plus one or two hollow cylindrical insulators, made of an electrically insulating material, such as a ceramic, a suitable polymer, glass or the like. As a rule, the insulator in a two-electrode surge arrester is soldered to the end electrodes at two sides, joining them vacuum tight.
One method of producing a conventional surge arrester is outlined, for example, in U.S. Pat. No. 4,437,845. According to U.S. Pat. No. 4,437,845, the manufacturing process consists of sealing at a suitable temperature the components of the tube at substantially atmospheric pressure in a light gas mixed with another gas which, in view of the intended function of the tube, is desirable and heavier than the first-mentioned gas, and reducing the pressure exteriorally of the tube below atmospheric pressure, while simultaneously lowering the temperature to such extent that the heavy gas can only to an insignificant degree penetrate the tube walls through diffusion and/or effusion, and the enclosed light gas can diffuse and/or be effused through the walls such that, as a result of the pressure difference, it will exit through the walls of the tube, thus causing a reduction in the total gas pressure inside the tube.
Further, an outside coating of the surge arrester components has been disclosed in U.S. Pat. No. 5,103,135, wherein a tin coating is applied to the electrodes, and an annular protective coating is applied to the ceramic insulator having a thickness of at least 1 mm. This protective coating is formed from an acid-resistant and heat-resistant colorant or varnish which is continuous in the axial direction of the surge arrester. The protective coating may form part of the identification of the surge arrester. For example, the identification may be in the form of a reverse imprint in the protective coating. In addition, tin-coated leads can be coupled to the electrodes.
U.S. Pat. No. 4,672,259 discloses a power spark gap for protection of electrical equipment against overvoltages and having high current capacity, which spark gap comprises two carbon electrodes each having a hemispherical configuration and an insulating porcelain housing, whereby the carbon electrodes contains vent holes to the inner thereof to provide arc transfer to an inner durable electrode material. The spark gap is intended for high voltage lines, wherein the expected spark length is about 2.5 cm (1 inch), transferring 140 kV or so. This spark gap is not of the type being hermetically sealed and gas filled, but communicates freely with the air. The arc formed starts from the respective underlying electrodes and passes the vent holes. Thus the formation of the spark is, to a great part, based on the underlying material, which is not necessarily inert, but is due to oxidation in the existing environment, which means that the spark voltage can not be determined, and reproduced.
U.S. Pat. No. 4,407,849 discloses a spark gap device and in particular a coating on the electrodes of such spark gap, in order to minimize filament formation. The coating is applied onto an underlying electrode, whereby the coating may consist of carbon in the form of graphite. The surge limiter is a gas filled one. The reference does not address the issue of having an inert surface or not on the electrode, or any problems related thereto.
The previously mentioned problems of sensitivity and recovery have been addressed by the use of an electron donor on the electrode surfaces or elsewhere. This electron donor can comprise radioactive elements, such as tritium and/or toxic alkali metals, such as barium. It is obvious, that this solution has specific drawbacks associated inter alia with the radioactivity and/or toxicity of the components.